To enable support for importing and exporting fbx and obj files, you'll need a set of C# wrappers around Autodesk's FBX sdk, and some support functions implemented in C for speed.

1. Clone Unity's com.autodesk.fbx package
2. (optional) Follow the build instructions to make sure you have all the other pieces you need (cmake, swig, pcre, ...) before the next step. NOTE: Although the instructions say to install VS 2015, the makefile expects VS 2017, so install that instead.
3. Copy tilt_brush.i into the Source directory of your com.autodesk.fbx repository and apply the patch com.autodesk.fbx.patch.
4. Follow the build instructions in com.autodesk.fbx. If successful, this will create a directory build/install/com.autodesk.fbx.
5. Add the package to your Tilt Brush project. The quickest way to do this is to copy or link the resulting directory build/install/com.autodesk.fbx into the Packages directory of your Tilt Brush repository.
6. Add FBX_SUPPORTED to Project Settings -> Player -> Scripting Define Symbols.